Recently, with the development of optical technology, various display technologies for replacing a conventional cathode ray tube (CRT) have been suggested, and various types of display devices such as plasma display panels (PDPs), liquid crystal displays (LCDs), organic/inorganic EL displays (ELDs) and the like have been commercially available on the markets. In the above-mentioned displays, the use of various types of plastic films has been suggested and the required characteristics thereof have been sophisticated. For example, in order to achieve slim and lightweight LCDs and improve display characteristics, various types of plastic films are used in a polarizing plate, a retardation film, a plastic substrate, a light guide plate, and the like.
In general, the polarizing plate has a structure in which a triacetyl cellulose film (hereinafter, referred to as a “TAC film”), acting as a protective film, is laminated onto the polarizer by an aqueous adhesive made of a polyvinyl alcohol-based aqueous solution. However, both the polyvinyl alcohol film used as the polarizer and the TAC film used as the polarizer protective film have poor heat resistance and moisture resistance. Therefore, if the polarizing plate that is made of these films is used in a high-temperature or high-humidity atmosphere for a long period of time, the degree of polarization thereof will be reduced, the polarizer and the protective film will be separated from each other, and the optical properties thereof will be reduced. Thus, such a polarizing plate has various limitations in its applications. In addition, in the case of the TAC film, in-plane retardation (Rin) and thickness retardation (Rth) are greatly changed according to a change in the ambient temperature/humidity environment. In particular, a change in retardation with respect to incident light in an inclined direction is great. If the polarizing plate that includes the TAC film having the above-mentioned characteristics is applied to an LCD as the protective film, there will be a problem in that the viewing angle characteristics are changed according to the change in the ambient temperature/humidity environment, thus reducing image quality. In addition, in the case of the TAC film, the change in dimension according to the change in the ambient temperature/humidity environment is great and the photoelastic coefficient is also relatively high, and thus, after evaluation of durability in a heat-resistant and moisture-resistant environment, a change in retardation characteristics occurs locally, thereby adversely affecting image quality.
As a material to complement the disadvantages of the TAC film, a methacrylic resin is well known. However, it is known that the methacrylic resin has a property that it is easily broken or split. And it is also known that the property causes problems in terms of transportability during the production of the polarizing plate and resulting in poor productivity. In addition, in the case in which acrylate resin is used as the material of the film, there is a problem in that it is required to use a casting method which is both costly and difficult to perform.
In the case of a retardation film, a film made of a styrene-based resin is a material showing the optical anisotropic property that the refractive index is increased perpendicular to the alignment direction when the film is stretched and aligned. It is known that the styrene-based resin is a material useful for manufacturing a film having the positive thickness retardation (Rth) by a stretching process. In addition, the styrene-based resin has the advantages of excellent economic efficiency and transparency. However, the styrene-based resin has the problems of insufficient heat resistance and poor mechanical properties, except for the case in which it is prepared using a costly special monomer. In addition, in the case in which a retardation film is manufactured by stretching a polycarbonate resin, it is possible to provide a sufficient retardation function; however, the change in retardation with respect to the degree of stretching is great and it is difficult to provide a film having a uniform and stable retardation function.